1. Field of the Invention
The present invention relates to electro active devices, and in particular, to a directional flextensional transducer.
2. Description of the Prior Art
Electro active devices in the form of flextensional transducers were first developed in the 1920s and have been found to be particularly useful for underwater acoustic detection and transmission since the 1950s. They typically comprise an active piezoelectric or magnetostrictive drive element coupled to a mechanical shell structure. The shell is used as a mechanical transformer which transforms the high impedance, small extensional motion of the ceramic into a low-impedance, large flexural motion of the shell. The term xe2x80x9cflextensionalxe2x80x9d is derived from the concept of the extensional and contractional vibration of the drive element causing a flexural vibration of the shell. Flextensional transducers have been divided into seven classes according to the shape of the shell and the configuration of the drive elements. For example, a Class I transducer has a shell similar to an American football in shape. The drive motor is typically a stack of drive elements oriented along the major axis of the shell. A Class II transducer is essentially a modified Class I shape having extensions along the major axis. A Class V transducer, applicable to this application, typically includes a radially vibrating ring or disk as a drive element, as opposed to a linear stack of drive elements oriented along a major axis of the shell. The radially vibrating ring or disk is usually sandwiched between two spherical cap shells.
Flextensional transducers may range in size from several centimeters to several meters in length and can weigh up to hundreds of kilograms. They are commonly used in the frequency range of 300 to 3000 Hz. Such transducers can operate at high hydrostatic pressures, and have wide bandwidths with high power output.
Two electro active devices, versions of the Class V flextensional transducer, called the xe2x80x9cmooniexe2x80x9d and the xe2x80x9cCymbal(trademark)xe2x80x9d have been developed at the Materials Research Laboratory at the Pennsylvania State University (Cymbal(trademark) is a trademark of the Pennsylvania State University). The moonie and Cymbal(trademark) can be constructed using bonding and fabrication processes that are very simple, therefore, they can be inexpensive and easy to mass-produce.
An example of a moonie transducer is described in U.S. Pat. No. 4,999,819. The moonie acoustic transducer utilizes a sandwich construction and is particularly useful for the transformation of hydrostatic pressures to electrical signals.
U.S. Pat. No. 5,276,657 describes a moonie ceramic actuator similar to that shown in FIG. 1. A piezoelectric or electrostrictive element 100 is sandwiched between a pair of endcaps 105, 110, with each endcap having a cavity 115, 120 formed adjacent to the piezoelectric element 100. The endcaps 105, 110 are bonded to the piezoelectric element 100 to provide a unitary structure. Conductive electrodes 125 and 130 are bonded to the piezoelectric element""s major surfaces. When a potential is applied between electrodes 125 and 130, the piezoelectric element 100 expands in its thickness dimension and contracts in its axial dimension, causing endcaps 110 and 105 to bow outward as shown by lines 135 and 140, respectively. The bowing action amplifies the actuation distance created by the contraction of the piezoelectric element 100, enabling the use of the element as an actuator.
U.S. Pat. No. 5,729,077 describes another Class V transducer having sheet metal caps with an outward convex shape, joined to opposed planar surfaces of the ceramic substrate to improve the displacements achievable through actuation of the ceramic disk. Due to the shape of the sheet metal caps, the transducer is commonly known as a Cymbal(trademark) transducer, as mentioned above. An example of a Cymbal(trademark) transducer is shown in FIG. 2. A multi-layer ceramic substrate 200 is interposed between two end caps 205 and 210. The multi-layer substrate 200 includes a plurality of interspersed electrodes 215 and 220. Electrodes 215 are connected together by end conductor 225 to endcap 210 and electrodes 220 are connected together by end conductor 230 to endcap 205. Both endcaps are bonded to multi-layer substrate 200 about their periphery. Application of a potential across electrodes 215 and 220 causes an expansion of multi-layer substrate 200 in its thickness dimension, and contraction in its axial dimension, in a fashion similar to the moonie piezoelectric element 100 described above. As a result, endcaps 205 and 210 pivot about bend points 235, 240 and 245, 250, respectively. As a result of such pivoting, substantial displacement of end surfaces 255 and 260 occurs.
Thus, the structure of piezoelectric element 100 or multi-layer substrate 200 in combination with their respective endcaps convert and amplify the small radial displacement of the element or substrate into a much larger axial displacement normal to the surface of the caps. For underwater applications, this contributes to a much larger acoustic pressure output than would occur when using piezoelectric element 100 or multi-layer substrate 200 alone.
The moonie and Cymbal(trademark) transducers are capable of being constructed so as to be small compared to the wavelength of sound they produce in a usable frequency range, which is usually near their first resonance frequency. In addition, most of the radiating surface area of the shells moves in phase. As a result, the resulting acoustic radiation pattern is nearly omni directional, resembling an acoustic monopole. The omni directional characteristics of flextensional transducers create significant problems in projection transducer and array applications designed to transmit in one direction. At the present time, rows of transducers are carefully arranged and phased, or large baffles are used to produce the desired beam patterns. This is expensive, time-consuming and cumbersome. It would be desirable to construct and operate a Class V flextensional transducer that would be capable of generating a directional radiation pattern.
Butler et al., in xe2x80x9cA Low Frequency Directional Flextensional Transducer,xe2x80x9d J. Acoust. Soc. Am., vol. 102, July 1997, pp. 308-314, propose a method for generating a directional beam using a Class IV flextensional transducer by exciting both an extensional mode and a bending mode simultaneously. Butler et al. is directed to operating a Class IV transducer, in the 900 Hz range. The shell has an elliptical shape and the transducer is driven by a linear, rectangular stack of drive elements oriented along the major axis of the shell. The transducer disclosed by Butler et al. has overall dimensions of 19.4 inches long, 9.5 inches wide, and 20.3 inches high, and an in air weight of 350 lbs. In addition, Butler et al. discloses assembling six transducers in a line array with 20 inch center to center spacing. Thus the assembled array measures 10 feet long and weighs approximately 2100 lbs.
Prior to this application, there is no known method or apparatus for driving a Class V flextensional transducer to produce a directional beam.
An electro active device for generating a directional beam includes first and second electro active substrates each having first and second opposed continuous planar surfaces wherein each of the first opposed surfaces have a polarity and each of the second opposed surfaces have an opposite polarity. The first opposed surfaces of the first and second electro active substrates are in close contact. A first electrode is coupled to a junction formed by the first opposed surfaces having the same polarity, a second electrode is coupled to the second opposed surface of the first electro active substrate, and a third electrode is coupled to the second opposed surface of the second electro active substrate. A first endcap is joined to the second opposed surface of the first electro active substrate and a second endcap is joined to the second opposed surface of the second electro active substrate.
The first and second electro active substrates may be disc shaped, and the first opposed surfaces of the first and second electro active substrates may be bonded by a conductive layer to form the junction. The first and second electro active substrates may be formed of an electrostrictive material, and/or a piezoelectric material. If the substrates are formed of a piezoelectric material, the substrates may also be poled in a direction perpendicular to their first and second opposed planar surfaces.
The first and second endcaps may comprise a truncated conical shape and a rim portion. The rim portion of the first endcap may be joined to the second opposed surface of the first substrate, and the rim portion of the second endcap may be joined to the second opposed surface of the second substrate.
The electro active device may also include circuitry for applying a first electric field across the first and second electrodes, and circuitry for applying a second electric field across the first and third electrodes, where the second electrical field has a phase relationship with the first electrical field, and where the application of the first and second electrical fields causes the electro active device to produce a combined flexural and bending motion.
A vibration production system may be constructed from a plurality of the electro active devices by arranging the devices in an array.